1. Field of the Invention
This invention relates to a method of regenerating zeolite by heating. More particularly, it relates to a method in which zeolite having adsorbed (NH.sub.3 N--) in foul water is regenerated by heating.
2. Description of the Prior Art
Heretofore, the activated sludge process and the active carbon adsorption method have been mainly known as methods for disposing of industrial waste water and domestic waste water. With these methods, it is possible to remove organic matter contained in foul water and to turn the foul water into clear water. (NH.sub.3 --N), however, cannot be removed and is often emitted as it is. Since the NH.sub.3 --N serves as a supply source of nitrogen for algae, it causes the waste water to become nutritious and destroys the balance of growth of organisms. It becomes the cause of the occurrence of the so-called red tide and causes the death of fish in large quantities, and it becomes the source of offensive odors in rivers, lakes and marshes.
For this reason, the establishment of techniques for removing NH.sub.3 --N has been strongly desired. At present, there have been realized a variety of processes such as the ammonia stripping process, biological disposals, chemical treatments and adsorptions with zeolite etc.
Among these processes, the method of using zeolite including a natural zeolite such as clinoptilolite, mordenite and chabasite and a synthetic zeolite such as one manufactured by crystallizing the substances of raw materials serving as silica and alumina sources, wherein sewage or waste water containing NH.sub.3 --N is disposed of to adsorb and remove the NH.sub.3 --N from within the foul water is an excellent method because it is simple and it exhibits a high rate of removing NH.sub.3 --N. Further, when organic matter in the drain is simultaneously adsorbed by jointly using the active carbon adsorption, the disposal of the drain becomes more perfect.
In general, zeolite is represented by Me.sub.2/n O.Al.sub.2 O.sub.3.XSiO.sub.2.YH.sub.2 O (where Me denotes an alkali metal or an alkaline-earth metal, n denotes the charge of Me, and X and Y denote coefficients respectively). It is crystalline hydrous aluminosilicate of a three-dimensional structure made up of a tetrahedron of (SiO.sub.4) and (AlO.sub.4). The deficiency of charges caused by (AlO.sub.4) is filled up by the alkali metal or the alkaline-earth metal. Since the cations are exchangeable, the zeolite has the cation exchanging capability and selectively adsorbs and removes the cations in water. Among such zeolites, ones called clinoptilolite and mordenite selectively adsorb and remove especially ammonium ions (NH.sub.4.sup.+). Therefore, the inexpensive natural zeolites (clinoptilolite, mordenite, etc.) are often employed for the removal of NH.sub.3 --N in the foul water with the zeolite. As is well known, the active carbon adsorbs and removes especially organic matter into fine pores thereof by the Van der Waals force.
In case where the zeolite or the combination of the zeolite and the active carbon is employed for the removal of NH.sub.3 --N in foul water, the regeneration of the zeolite after the adsorption becomes a problem. In industry, it is desirable to adopt a cyclic system wherein the adsorbing operation and the operation of regenerating the zeolite having adsorbed NH.sub.3 --N are repeatedly carried out. An example of a regenerating method with such a cyclic system has heretofore been one wherein zeolite is packed in a packed column as in a pure water-producing equipment or a hard water-softening equipment which employs an ion exchange resin and which is used industrially a liquid to be treated is let to pass through the packed column so as to adsorb and remove NH.sub.3 --N, and thereafter, an aqueous solution of a hydroxide, carbonate, chloride or the like of an alkali metal or an alkaline-earth metal is let to pass therethrough. In another method as disclosed in the official gazette of Japanese Patent Application Laying -open No. 49-78676 or Japanese Patent Application Laying -open No. 50-31664, zeolite having adsorbed ammoniac nitrogen NH.sub.3 --N is heated to 400.degree.-600.degree. C., thereby to desorb the NH.sub.3 --N.
In the above methods for regenerating zeolite, the method wherein the solution of the hydroxide or the like of the alkali metal or the alkaline-earth metal is passed through the zeolite having absorbed NH.sub.3 --N has the disadvantages that a long time is required for the regeneration and waste fluid containing NH.sub.3 --N and a high concentration of pollutants is discharged, an after-treatment thereof being necessary. In contrast, the method of regeneration by heating wherein the zeolite having adsorbed NH.sub.4.sup.+ is heated to 400.degree.-600.degree. C. to cause the ammonia gas to escape can be said to be an excellent regenerating method because waste fluid at high concentration as described above is not discharged.
According to a study by the inventor, however, the method of regeneration of zeolite by heating involves problems to be stated hereunder. NH.sub.4.sup.+ on zeolite as ion-exchanged with cations (Na.sup.+, K.sup.+, etc.) in the zeolite as indicated in Eq. (1) given below escapes from the zeolite in the form of NH.sub.3 gas at temperatures of 250.degree.-300.degree. C. or higher as indicated in Eq. (2). The zeolite after regeneration adsorbs NH.sub.4.sup.+ in conformity with Eq. (3). In this case, however, at regenerating temperatures of 500.degree.-600.degree. C. or higher, it has been noted that a reaction of Eq. (4) given below takes place besides the reaction of Eq. (2), so any cation capable of adsorption in exchange for NH.sub.4.sup.+ does not exist on the zeolite subjected to regeneration by heating, with the result that the NH.sub.4.sup.+ adsorption capacity decreases conspicuously. In the above case, even when a favorable regeneration has been executed, the NH.sub.4.sup.+ adsorption capacity of the zeolite lowers after repeating the heating regeneration. This is considered to be attributed to the proceeding of the reaction of Eq. (4). EQU Z--O--Na+NH.sub.4.sup.+ .fwdarw.Z--O--NH.sub.4 +Na.sup.+ ( 1) EQU Z--O--NH.sub.4 .fwdarw.Z--OH+NH.sub.3.sup..uparw. ( 2) EQU Z--O--H+NH.sub.4.sup.+ .fwdarw.Z--O--NH.sub.4 +H.sup.+ ( 3) ##STR1## (Z represents the zeolite radical.)
Further, organic matters other than NH.sub.4.sup.+ coexist in the general waste water. Although zeolite does not present to the organic matter as high a selective adsorptivity as to NH.sub.4.sup.+, it adsorbs them to some extent. When the organic matter is in large quantity, it adheres to zeolite surfaces. In case of heating and regeneration the zeolite having adsorbed the NH.sub.4.sup.+ and the organic matter, NH.sub.4.sup.+ is desorbed as NH.sub.3 g as at 250.degree.-300.degree. C. as stated previously. In general, however, the organic matter other than NH.sub.4.sup.+ is not sufficiently burnt and desorbed unless heated to 400.degree.-600.degree. C. or above. The organic matter which has not been burnt and desorbed from the zeolite fills up the pores of the zeolite functioning for the adsorption of NH.sub.4.sup.+ and degrades the NH.sub.4.sup.+ adsorption capacity, so that the regeneration by heating of the zeolite must be generally carried out at about 400.degree.-600.degree. C. At such high temperatures, however, the lowering of the NH.sub.4.sup.+ adsorption capacity of the zeolite is inevitable. In particular, there is the problem that the lowering of performance at the repeated regenerations becomes conspicuous.
The foregoing removal of (NH.sub.3 --N) with zeolite is employed as the method for treating industrial waste water and domestic waste water. Accordingly, the foregoing method of active carbon adsorption is often used jointly in order to remove organic matter. In this case, the zeolite adsorbs cations in the water, especially ammonium ions, while the active carbon absorbs (physical adsorption) and removes especially the organic matter into its fine pores according to the Van der Waals force.
In the above case where the organic matter and the NH.sub.3 --N in the drain are absorbed and removed by jointly using the active carbon and the zeolite, it is desirable from the economical viewpoint to adopt a cyclic system wherein the adsorbing operation and the desorbing and regenerating operation are repeatedly conducted.
In the case of removing the organic matter and NH.sub.3 --N in the drain by jointly using the active carbon and the zeolite, the active carbon and the zeolite can be simultaneously regenerated, the simultaneous regeneration will become possible as well as the simultaneous removal of the organic matter and NH.sub.3 --N without separating the mixture into the active carbon and the zeolite. This is convenient in achieving the enhancement of the treating efficiency, the compaction of a treating equipment, etc.
The regeneration by heating of the active carbon needs to be done at or above 400.degree. C., preferably at 600.degree.-800.degree. C., in order to carbonize and decompose the adsorbed organic matter and to sufficiently regenerate the active carbon. On the other hand, the regeneration temperature of the zeolite is 300.degree.-600.degree. C., preferably 400.degree. C. or below, and when 600.degree. C. is exceeded, the ion exchange site of the zeolite collapses and the NH.sub.3 --N adsorptivity after the regeneration lowers conspicuously. Further, even in case of regeneration at or below 400.degree. C., the NH.sub.3 --N adsorptivity is gradually lowered by repeatedly executing the regenerations. Without any contrivance, therefore, the simultaneous regeneration by heating of the active carbon and the zeolite is impossible, and the active carbon and the zeolite cannot but be separated and regenerated individually.